1. Field of the Invention
The present invention relates to an apparatus for and a method of making an inflation film.
2. Description of the Related Art
The inflation molding technique has been widely employed in the manufacture of films made of a thermoplastic resin such as a polyolefin including a low density polyethylene, a high density polyethylene and so on, because an apparatus for conducting the inflation molding is simple and inexpensive. According to the general inflation molding, a molten thermoplastic resin is extruded through a circular die of an extruder and the extruded tubular film is drawn with an outer surface thereof cooled by a stream of air blown off from a cooling ring, while, at the same time, a gaseous medium such as air is introduced into an inner space of the tubular film so that the tubular film can be expanded to a predetermined size by the effect of the pressure of the gaseous medium, and the expanded tubular film is then passed through a pair of nipping rolls to flatten the tubular film to give a folded sheet. Thereafter, the folded sheet, that is the flattened tubular film, may be converted into two separate flat films with the lug of the folded sheet cut off by a slitter. The flattened tubular film or the resulting flat films are then wound up around a take-up roll or reel.
With the inflation molding technique discussed above, the tubular film extruded from the extruder through the circular die is stretched in a longitudinal direction thereof and also in a direction transverse to the longitudinal direction by the effect of a take-up and an expansion. Also, at the same time, the film transits from a molten state to a solid state by cooling and, during the course of the cooling, the physical properties thereof are fixed. Thus, in the inflation molding, the cooling process plays an important role in defining the physical properties of the resulting film.
Various attempts have been made to suppress the variation of the quality of the film obtained by the inflation molding. For example, the Japanese Patent Application Laid-open No. Hei 6-182868 (JPA 6-182868) discloses an inflation film making apparatus provided with a control means for controlling the quantity of a stream of air blown from the cooling ring so that the position of the frost line for the tubular film, extruded from a circular die, can be kept at a predetermined height above the circular die. The frost line referred to above is a process line at which the tubular film is solidified, by cooling, to have a designed diameter.
The Japanese Patent Application Laid-open No. Hei 6-39916 (JPA 6-39916) discloses an inflation film making method and an apparatus for use in this method in which, in addition to a first air ring disposed in the vicinity of a circular die, a second air ring having a plurality of annular slits is provided at a position where a bubble of a thermoplastic resin extruded from the circular die undergoes a rapid expansion. According to this apparatus, cooling of the bubble is effected by the air blown from the first air ring and the cooling condition is adjusted so that the temperature of the bubble, when reaching the position of the second air ring, can fall within the predetermined temperature range.
It has, however, been found that, according to the inflation film making apparatus disclosed in JPA 6-182868, although the control for the frost line position can effectively work to a certain extent, the formed film tends to exhibit a variation in film thickness and occurrence of slack and wrinkles, all of which are caused by a fluttering motion of the bubble, that is the tubular film in a molten state.
On the other hand, JPA 6-39916 describes that the second air ring is preferably so constructed as to blow off the cooling air in a direction generally parallel to the direction in which the bubble is drawn or towards the outside of the bubble and that “the atmosphere of reduced pressure created by the blow of the cooling air directly affects the bubble enough to cause a rapid expansion of the bubble at this position”. This method is intended to control the position at which the expansion of the bubble starts, in order to suppress the instability of the bubble formed by inflation molding with a long neck type even where a thermoplastic resin of a low melt tension is employed.
However, although JPA 6-39916 discloses that the temperature of the bubble at the position of the second air ring is adjusted to fall within the predetermined temperature range, it does not address the variation of the temperature of the bubble. Also, the air blown from the second air ring is described to create the atmosphere of reduced pressure, but this is likely to cause an abrupt change in the manner of the bubble being cooled. Therefore, the method disclosed in JPA 6-39916 requires a precise adjustment of the operating condition in order to suppress the change in the quality of the formed film.